Trailer specifies additional headers that are sent after the request body.

For server requests the Trailer map initially contains only the trailer keys, with nil values. (The client declares which trailers it will later send.) While the handler is reading from Body, it must not reference Trailer. After reading from Body returns EOF, Trailer can be read again and will contain non-nil values, if they were sent by the client.

For client requests Trailer must be initialized to a map containing the trailer keys to later send. The values may be nil or their final values. The ContentLength must be 0 or -1, to send a chunked request. After the HTTP request is sent the map values can be updated while the request body is read. Once the body returns EOF, the caller must not mutate Trailer.

Few HTTP clients, servers, or proxies support HTTP trailers.